This invention relates to medium/high carbon low alloy steels which exhibit excellent cold/warm formability and subsequent heat treatment characteristics.
Articles such as hand tools and more particularly sockets for socket wrenches or needle-nosed pliers are usually fabricated of a steel, shaped by severe cold/warm forming (e.g. cold or warm forging at temperatures between about ambient and 1,600.degree. F.) and then heat treated to achieve the desired final properties of hardness, strength, ductility and toughness. Ductility or strain-related characteristics of a material are related to both formability and toughness. Toughness is related to the ability of the material to absorb energy which can be related in turn to ductility and strength, reduction in area, impact toughness or fracture toughness.
Articles of the foregoing type may be manufactured in large quantities, need to exhibit a low fracture or failure rate in forming, should maintain dimensional tolerances and exhibit dimensional stability during forming and heat treatment, must provide the desired combination of final properties after heat treatment, must perform in the intended application and must exhibit high customer or user acceptance.
Hand tools such as needle-nosed pliers and wrench sockets are frequently used in automotive and industrial settings where impact loads and/or bending and/or wear are common occurrences. Plier tips which are slender and have gripping notches or teeth may fracture at the tip due to bending stresses particularly in the presence of notches or teeth. Moreover, sockets may be improperly used or overloaded and thus fracture, fail or wear prematurely. Furthermore, any new material should be capable of being formed and heat treated using present forming and thermal treating equipment and techniques.
Steels with a carbon content of less than about 0.30 weight percent are generally considered to be formable (after softening by the process known as spherodizing) by cold forming into various shapes. Some shapes have very small sections (e.g. thin wall and tips) into which the metal must flow and thus require particularly severe formation. Low carbon steels exhibit the required formability. However, some low carbon low alloy steels cannot be heat treated to exhibit acceptable properties in terms of hardness, strength, ductility and toughness.
But, other low carbon low alloy steels have been used in the manufacture of wrench sockets. See for example U.S. Pat. Nos. 4,319,934; 4,332,274; and 4,322,253. Parts made from the disclosed steels can be subsequently heat treated to between 48 and 50 Rockwell C (R.sub.c) which suggests its wear and strength properties.
In the manufacture of needle-nosed pliers, medium/high carbon steels have been used to achieve the desired strength at the tip where substantial bending forces are incurred. But in the presence of notches (such as serrations or teeth) the present material does not exhibit sufficient toughness or the ability to absorb impacts. This material is hardened to achieve the higher strength for bending resistance or hardness for wire cutting edges that are provided. Thus, while the material must be hardenable, it must also be formable, tough and resist bending forces.
It is desirable to provide sockets, pliers and the like which exhibit greater wear, toughness and strength properties while still being formable by techniques and equipment developed and presently used. However, it is believed that the only way to achieve those greater properties is to provide a steel where the carbon content is increased to a level considered too high for acceptable part formation, without failure or fracture of the parts, without significant modification to existing part forming processes and equipment and without significant distortion.
It is generally known that high carbon steels can be heat treated to higher strength and hardness levels than low carbon steels but that high carbon steels are difficult to cold work or form especially in the same applications and by the same techniques as for the low carbon steels. Moreover, high carbon steels may not exhibit the desired ductility, toughness or formability.
There are some disclosures of steels that have a carbon content up to about 0.60 weight percent which are said to be useful in some cold forming processes of limited severity. See for example U.S. Pat. Nos. 3,489,620 and 4,326,886. However, materials so disclosed have not been found to have been used in the foregoing applications or for the foregoing types of parts. Reasons for nonuse may include the inability of the material to exhibit the required combination of properties (e.g. formability, strength, and toughness), availability, cost, etc.
Thus it is an object of this invention to provide a steel or family of related materials which can be used in socket-wrenches, needle-nosed pliers or similar parts which require severe cold/warm working, which material minimizes fracture of failure, which can be heat treated to enhanced properties such as hardness, toughness and strength and which material also minimizes part distortion.
This and other objects of this invention will become apparent from the following description and appended claims.